24 INTRODUCTION. 
Thus nerves are the messengers by which the mind receives 
information of the external world (nwnti? rerum). 
But the nerves are equally the ministers of the will, which by 
their assistance is able to act upon the muscles. By Muscles are 
understood those active organs of motion (organa motus activa) 
which are fixed to other parts, as their point of resistance, and 
these last are called passive organs of motion (organa motus passiva). 
The harder fibres, which serve for the insertion of muscles, form 
Tendons, of which the colour in animals with red flesh, as in man, 
is white. In many animals the muscles are inserted into the skin, 
or into certain hard portions of the skin, as in Insects, whose hard 
and often horny coverings supply the place of a skeleton in that 
respect. A skeleton is, properly, a connected whole of imternal 
passive organs of motion—cartilagmous or bony, and these serve 
not only for motion, but moreover, and indeed especially, for the 
protection of the most important parts of the nervous system, the 
Brain and Spinal Cord. The skull (for the protection of the Brain) 
and the Vertebral Column (which encloses the Spinal Cord) must 
therefore be considered as the principal parts of the skeleton, of 
which ribs and limbs are only appendages: in this simple condi- 
tion is the skeleton met with in the Larva, for example, of the 
Frog. 
Development of Animals. 
How the expression imperfect Animal is to be understood. 
We have attempted to give a general idea of the organs which 
compose the animal body. But these organs are by no means 
found in all animals. Only in the more perfect animals is the 
structure thus complicated. When from these we descend in the 
animal scale, we perceive in the long series one instrument after 
another gradually decrease in magnitude and development, and at 
last entirely disappear. In Polyps (Aydre) nothing remains but 
the Intestinal Canal. The entire animal forms a blind sac com- 
posed of a single tissue, and all the vital functions which it 
performs are effected through one and the same gelatinous mass. 
Finally, in some Infusories we no longer perceive even an intestinal 
canal—nothing remains but an homogeneous gelatinous body, whose 
